Grimaldi's wonderful skidding strike in the 91st minute was enough to beat the otherwise outstanding Patrick Beach, to send AAMI Park into raptures and deliver the Victory's first derby win since October 2024.
Juan Mata's deflected shot forced a late corner and the Spanish maestro took it quickly and played it short to Denis Genreau, who worked it to Grimaldi.
The young gun, who joined late in pre-season after Western United went into hibernation, took two touches then drilled a left-footed shot inside the near post.
"Unbelievable. Honestly, I can't even remember the ball hitting the net. It's still a bit of a blur," Grimaldi said.
"I've grown up supporting this club and didn't get to play for their academy and go through the ranks.
"I ended up at City and Western now it's a full circle moment, and I get to score in a derby. I'm just absolutely buzzing."
Coach Arthur Diles hailed Spanish star Mata's instincts.
"He's gone over there quickly to take it. He can smell it. He can feel it," he said.
"And then Matty does his business, which was a fantastic strike."
A second win on the bounce moves Victory up to seventh.
In a feisty, physical Saturday night clash, former Victory coaches: ex-Tottenham and Nottingham Forest boss Postecoglou and Shanghai Port mentor Muscat, headlined a plethora of big football names.
Postecoglou and Muscat were among a group that headed into the Victory rooms - which Grimaldi missed as he was celebrating with fans.
"I just missed Ange, so I'm gutted, but it's pretty cool to know he was here watching," Grimaldi said.
Diles added: "They're incredible human beings. They're fantastic coaches, and they've represented this club and been very successful with this club.
"So just to have them in our change rooms and in our presence is fantastic for our club, fantastic for me and my staff, and hopefully the players appreciate that as well."
Former Socceroos defender Craig Moore, ex-Victory star Besart Berisha, Socceroos assistant coach Hayden Foxe and Matildas coach Joe Montemurro were all on hand too.
Brighton & Hove Albion owner Tony Bloom, part-owner of the Victory, was also present, as was Hollywood star Gerard Butler.
The two goalkeepers, City's newly-capped Socceroo Beach and Victory veteran Jack Duncan, both made multiple exceptional saves.
Victory striker Nikos Vergos spurned golden chances in the 12th and 15th minutes, while champions City worked their way into the game.
On the half-hour mark, Duncan brilliantly used his leg to deflect a Max Caputo strike onto the crossbar before the offside flag went up regardless.
Beach made a terrific one-handed strike to deny a fierce Vergos strike in the 55th minute.
Four minutes later, Caputo clattered a header off the bar.
Duncan produced an extraordinary reflex body save to block away Medin Memeti's header on 80 minutes.
City will lick their wounds then host Macarthur FC on Tuesday, while the Victory don't play again until hosting Wellington on December 29.
"It was probably one of those games that probably could have gone either way - and unfortunately for us, it went the wrong way," coach Aurelio Vidmar said.
"We have to suck it up - that's how it is and got to move forward.
"Don't have much time now. We've got another game on Tuesday against Macarthur. So lick the wounds and move on."
